This invention relates to a process for employing ion exchange resins suitable for absorbing precious metal ions, particularly gold ions from aqueous solutions.
By the practice of the Merril-Crow process, precious metals are conventionally extracted from ore by grinding the ore to a fine powder in a water slurry and leaching the metal from the ore with a cyanide and oxygen mixture. The resulting metal cyanide complex, which is soluble in water, is obtained as a filtrate as the ore is collected. The metal laden filtrate is then reduced using zinc metal. The metal which is recovered can be smelted and purified.
Precious metals can also be recovered by providing a metal cyanide complex as described hereinbefore and adsorbing the complex on activated carbon. The loaded carbon can be screened from the pulp or leach stream and the metal recovered from the carbon by an electroelution in hot caustic/cyanide solution. The metal can then be smelted and purified. A carbon-in-pulp process eliminates expensive filtration and clarification equipment required in the Merril-Crow process.
Although the aforementioned processes provide a means for recovering precious metals from ores, it is desirable to provide improved processes for such recovery. In particular, it would be desirable to provide a process for recovering precious metals which does not require expensive filtration/clarification equipment, utilizes an ion exchange resin which is substantially resistant to fouling, can easily be eluted and regenerated, is highly selective, and is capable of providing recovery of precious metals in high yield.
Processes for the recovery of gold and other precious metal values from ores using ion exchange resins are described in U.S. Pat. No. 2,648,601; Green, B. R. and Potgeiter, A. H., Council for Mineral Technology, South Africa, "Unconventional Weak-Base Anion Exchange Resins, Useful for the Extraction of Metals, Especially Gold", Ion Exchange Technology, D. Naden and M. Streat, eds., London Society Chemical Industry, 1984; Fleming, C. A., Council for Mineral Technology, South Africa, "Some Aspects of the Chemistry of Carbon-in-Pulp and Resin-in-Pulp Processes", the Aus. I.M.M. Perth and Kalgoorlie Branches and Murdoch University, Carbon-In-Pulp Seminar, July, 1982; and Mehmet, A. and Te Riele, W.A.M., Council for Mineral Technology, South Africa, "The Recovery of Gold from Cyanide Liquors in a Counter-Current Contactor Using Ion Exchange Resin", Ion Exchange Technology, D. Naden and M. Streat, eds., London Society Chemical Industry, 1984. The conventional ion exchange processes for recovery of precious metals involve dissolving the metal values by cyanidization, providing a solution in a slightly basic pH range, sorbing the dissolved values on a resinous weak-base anion exchanger, separating the exchanger from solution, and desorbing the values from the exchanger at a pH range of about 13 to 14. A limitation of the disclosed processes arises due to the fact that typical mine streams operate at a pH of about 10 to about 11. At such high pH values, the conventional weak-base resins, disclosed in the aforementioned references, do not attain a high loading capacity or are not chemically stable at such pH values. The prior art generally avoids the loading problem by lowering the pH value of the mine streams.
In view of the deficiencies of the prior art, it would be highly desirable to provide an improved process for recovering precious metals from aqueous solution at a relatively high pH range. The process is advantageously effective and efficient, and if an ion exchange resin is employed, the resin should be easily and efficiently regenerated.